An artificial membrane system is used as a well defined model to study in vivo molecular assembly and function. Monomolecular films are formed at an air-water interface using fragments of our rod segments, various rhodopsin preparations, opsin and retinal. The effect of light on their surface properties are measured. The parameters measured are molecular area, surface potential, dipole moment, absorption spectra, index of refraction and film thickness. The project is concerned with the critical testing of hypotheses of the primary photophysical processes of vision.